While the couplings certainly are a factor for heavy (bulk) freight (steel, coal, ore, aggregates, etc.), I'd guess that container trains are often rather length-limited by all the rest of the infrastructure (yard/siding/loop length, signalling) before hitting the actual limits of the old-style screw couplings. Whereas e.g. most of Europe currently has evolved towards a maximum length of around 750 m (which doesn't mean that the infrastructure in terms of loop/sidings length actually allows trains that long everywhere, though!), Denmark allows up to 835 m while still using screw couplings.

So for container trains at least, a programme of loops and sidings lengthening would be the more pressing requirement, before actually and absolutely having to upgrade the couplers, but that kind of infrastructure modification doesn't come cheap or necessarily easy, either.

Another difference to consider is that in Europe, passenger traffic is much more widespread and common than on North American railways, and also commonly has priority over freight trains.

While in principle, longer trains are indeed more efficient than an equivalent larger number of shorter trains, past a certain point they'll eventually become too unwieldy to fit inside the schedule demands of passenger traffic. There's certainly scope for moving beyond the current 750 m maximum, but the multi-kilometre range would probably be pushing things a little too far without completely dedicated infrastructure.